Every arc it feels like we get speculation about the new antagonist where people speculate they're more of an anomaly than they actually are.
Some recent examples:
"Kaido is a dragon that ate a human devil fruit"
"Kaido is actually a Poneglyph that was fed a fruit and that's why he's so durable" (in retrospect this is one of the most bizzare theories of all time)
"Kaido's fruit is called the Uo Uo no Mi because originally he was a fish, and the dragon is his awakened mode"
Reality:
Kaido... ate a dragon devil fruit.

We're seeing the same thing now:
"Saturn is a demon that ate a human devil fruit/takes a human form"
"Saturn has demonic powers that we have never seen before, these are not from a devil's fruit"
"Saturn's abilities are from haki, his regeneration is a reverse haki technique" (saw a JJK fan say this lmfao)

If we follow the same pattern and apply it here:
Saturn... probably just ate a Gyuuki fruit... and had the immortality surgery
I'm not saying the above theories are impossible or anything like that, but just don't forget occam's razor: the simple and obvious explanation is usually the right one.
Some recent examples:
"Kaido is a dragon that ate a human devil fruit"
"Kaido is actually a Poneglyph that was fed a fruit and that's why he's so durable" (in retrospect this is one of the most bizzare theories of all time)
"Kaido's fruit is called the Uo Uo no Mi because originally he was a fish, and the dragon is his awakened mode"
Reality:
Kaido... ate a dragon devil fruit.

We're seeing the same thing now:
"Saturn is a demon that ate a human devil fruit/takes a human form"
"Saturn has demonic powers that we have never seen before, these are not from a devil's fruit"
"Saturn's abilities are from haki, his regeneration is a reverse haki technique" (saw a JJK fan say this lmfao)

If we follow the same pattern and apply it here:
Saturn... probably just ate a Gyuuki fruit... and had the immortality surgery
I'm not saying the above theories are impossible or anything like that, but just don't forget occam's razor: the simple and obvious explanation is usually the right one.